Power supplies such as notebook adapters, wall adapters, and car adapters incur power losses when voltage is converted from one form or level to another form or level, because such conversion processes are less than one hundred percent efficient. The power losses result in heating of the power supply, and the heat that must be dissipated to the environment. Due to regulatory and marketing considerations, the temperature rise that can be tolerated for such power supplies is usually related to a maximum allowable surface temperature, such as 85 degrees Celsius, and a design ambient environmental temperature, such as 40 degrees Celsius. One way of meeting such limitations is to increase the size of power supplies in order to provide greater surface area for the dissipation of heat.
Another way of meeting such limitations without increasing the size of power supplies is use forced air or active cooling in which cool air is moved over the heat producing components and the resulting warm air is then vented away from the main body of the power supply. One problem with using forced air or active cooling is that it is usually accomplished by the use of a fan, which uses energy and therefore reduces efficient, and which can be noisy, expensive, can require a significant amount of space, and can have a short lifetime.
Another way to meet such limitations is to improve the efficiency of the power supply, such as by reducing the on resistance of MOSFETs or placing multiple diodes in parallel to reduce power losses. While these solutions provide some measure of improvement, one of the largest producers of heat for power supplies is the main transformer. Faraday's laws govern the efficiency that can be obtained by the transformer and thus the heat produced. The cooler a transformer is, the cooler a power supply will become. Unfortunately, these prior solutions do not address the main source of heat, which is the main transformer.